1. Field of the Invention
The present invention relates to the provision of a supply voltage to a drive amplifier, the drive amplifier providing a drive signal to a winding of a transformer. The invention is concerned particularly, but not exclusively, to an arrangement in which the drive amplifier is a correction amplifier forming a correction path of a control loop, a transformer being used to combine the output of the correction path with an output of a main path. The invention is further particularly, but not exclusively, concerned with such an arrangement providing a modulated supply voltage to an RF (radio frequency) amplifier, the modulated supply voltage being provided by the combined output of the transformer.
2. Background to the Invention
Key amplifier characteristics such as load impedance, supply voltage and peak efficiency are determined by an amplifier's maximum output power requirements. If a power amplifier, such as an RF amplifier, is operated at less than maximum output power its efficiency is reduced. When amplifying a high dynamic range signal, the power amplifier typically achieves maximum output power only rarely and frequently operates at significantly lower power. Hence, the power amplifier may exhibit low overall efficiency.
Various techniques are known in the art for enhancing power amplifier efficiency based on the supply voltage. One broad classification of solution is envelope tracking.
In a known envelope tracking technique an efficient switched mode supply stage, comprising a variable pulse width modulator, may be combined with a linear correction amplifier. The efficient switched mode supply provides a coarse approximation of the output signal, containing a majority of the required power, and the linear correction amplifier provides a high bandwidth correction signal which is combined with the coarse approximation signal. A modulated power supply with high bandwidth and generally good efficiency is thereby provided.
An example of an RF amplification stage incorporating a particularly advantageous technique in accordance with these principles is disclosed in British Patent No. 2398648.
In general, in such a control system, a coarse or low frequency path is fed by a control signal which is low-pass filtered and then used to control a switched mode power supply that provides the coarse output. In a correction path, a feedback signal is received from the output. The feedback signal is compared with an ideal reference to produce an error signal. A linear correction amplifier in the correction path provides a high frequency correction output from the error signal. The correction output is combined with the coarse output to provide an error-corrected output. The error-corrected output provides the modulated supply voltage.
Overall such an arrangement, particularly when used for providing a modulated power supply, provides high efficiency and high bandwidth amplification simultaneously.
However inefficiencies arise in respect of certain components within the linear correction amplifier. The linear correction amplifier must operate over a wide range of potential voltages, up to a possible peak voltage. The linear correction amplifier has to provide a large negative correction voltage during periods of high voltage from the coarse output. Conversely a large positive correction voltage is required during periods of low voltage from the coarse output. If the supply voltage in the linear correction amplifier is fixed, it has to be set to a sufficient value for the amplifier to provide any peak output levels without clipping. This requires the amplifier to have a higher supply voltage than necessary most of the time, and consequently its efficiency is degraded.
It is an aim of the invention to address the above-stated problem.
In particular it is an aim of the invention to provide a technique in which the voltage drop across the output device of the correction amplifier is reduced. It is an aim of the invention particularly to provide such an improvement in an arrangement in which a transformer is used to combine the outputs of a coarse path and a correction path.